A subdomain is an additional part of your primary domain. Please, follow this tutorial to remove a subdomain in Plesk.
1. Log in to your Plesk account.
2. Click on Websites & Domains from the Sidebar Menu.
3. Under the list of Domain names, click on the More icon 
in front of the subdomain and select Remove subdomain.
4. Confirm with Yes to remove the subdomain.
The subdomain has been successfully removed in Plesk.